Telecommunication system



G. F. BELLAIRS ETAI. 2,607,853

TELECOMMUNICATION SYSTEM Aug. 19, 1952 Filed Jan. 24, 1948 7Sheets-Sheet v1 oEMoDuLAToR yxER LFA/ MIB EB I ow FREQUENCY AMPLIFIERFigli- A Fi .l i Z 3 Eg @www Aug. 19, 1952 G. F. BELLAIRS ETALTELECOMMUNICATION SYSTEM Filed Jan. 24, 1948 7 Sheets-Sheet 2 mA /M-'XERA v Dc A2' "ow PASS DETECTOR B E R F02 /lwxER ILTE l M2@ Y M25 2 RELAYFXED 1 T52 [PFZ D B osclLLAToR |NTERMED|ATE FREQUENCY B MIXER,AMPLInElER 6C T25 L MeA /FA2 02A DETEcToR RELAY B, D5, ELLA mscRlMlNAToR-VARIABLE Y REAcTANcE FREQuENcY/- W02 CD vALvE osclLLAToR swEEP soosclLLAToR Aug. 19, 1952 G. F. BELLAlRs ETAL TELECOMMUNICATION SYSTEMFiled Jan. 24, 1948 7 Sheets-Sheet 3 Aug. 19, 1952 y s. F. BELLAIRS ETAL 2,607,853

I TELECOMMUNICATION SYSTEM Filed Jap. 24, 1948 7 sheets-sheet 4 DE@ @DlrhTAEaR-IIIEIQTE FREQUENCY La A+ 664B @ie-E- D4D -QHE Ma /fa Maf MlxERL MlxER CuETEcToR MARKER AB MBI- PMA /M'XER osclLLAToR TAZ 64 I l 74A MwLow PAss -g DI H FILTER J M4f 4 DA1 Pm Moz osFcl ILELTR F04 MIC B IDETECTOR I BAND TA] AB PAss K FILTER Mal-1 E MIXER 43A v 'MIXER E IBA04A M44 /MA p4A M40 E l VDETECTOR L l @-21155 MIXER Bca FREQUENCYAMPLIFIER 0F46 0 L INTERMEDIATE/44.

I I I l T Al BAND PASS L glsfyld FILTER MHD BAI '||-JAH ssa MHD Mr/v BcE MTN Aal Mu ,I .nu

TM5 .LUM/f1.0

E @LWN 4 @wf/M4 Aug. 19, 1952 G. F. BELLAlRs ETAL TELECOMMUNICATIONSYSTEM 7 Sheets-Sheet 5 Filed Jan. 24, 1948 Aug. 19, 1952 Filed Jan. 24,1948 G. F. BELLAIRS ET AL TELECOMMUNICATION SYSTEM 7 Sheets-Sheet 6 JfcaJc JfcA Jaa/I @mA-5h 4 -lsc ascAfh Q-0sc5 MATCHING MATCHING NETwoRK MN5AMN5B -NETwoRK IIIII --IL-II---H TERMINATING I (LQSHCNE EQUIPMENT`|` 755A065B I FREQUENCY I sELECToR BALANCING I NETwoRK w i I I l I I C 5x5 HMBl \HYBR|D CoIL I I dII-J I l I MECHANICAL Y MECHANICAL swITCIIINe l M5,.IfswITCHING EQUIPMENT 655A l 5?, EQUIPMENT I l I l l I l l l l I l l II 55A suBsCRIBI-:R s'rATIoN F 7 FTQ. 6 I 1Q- msA C665 T E frs/I Ivm/IFREQUENCY ffm ff7a fflgc* ma cas/I FINDER u, V "'U FREQUENCY WANTED-LINEF'NDER sELECToR 6B 56B FREQUENCY I W75 7b FREQUENCY FINDER I "T-T FINDERWANTED-LINE FQECYM SELECTOR GENERATOR. Z E

? MAW Aug- 19 1952 G. F. BELLAlRs E-r AL 2,607,853

TELECOMMUNICATION SYSTEM Fi1ed Jan. 24, 1948 'I sheets-sheet '7 8MULTlPLE WANTED-LINE SELECTOR 3,41 j MMSA 668A! L /L FREQUENCY 00211211n/sa Qilz MULTIPLE WANTED-LINE FLNCY frgb /sELEcToR C0852 c0052 ccs/aa14u/2580 a FREQUENCY F86 MULTIPLE WANTED-LINE FINDER /sELEcToR 000153605153 xvu, l

0 4 8 g. a l Q 4 8 l2 /6` L n o) 5 5 7 Q .C L F F 0 3 5 7 q /3 /5 dl n mn U11/'rs 1532 1020 1010 1000 1000 1502 1520 15 75 1552 1550 T2115 1052010200 15100 10020 10000 15020 1500015700 15520 15000' 11111100205 000020000 25000 20000 27200 20000 20000 20000 50000 31200 nection is eectedby mechanica1 or frequency switching and the frequency channel selectedleading towards the wanted exchange and to demodulating means thereat toenable a connection to be set up by mechanical or frequency switchingand to enable speech to take place between the calling and wantedexchanges. Telephone mains may thus be adapted for use between existingexchanges without necessarily having recourse to frequency switchingthroughout the system.

Present automatic telephone systems working on the step-by-stepprinciple involve the use of a multiplicity of switch contacts, bulkyapparatus and extensive trunking between the various groups of switcheswith consequent heavy costs. By employing high frequency distributionand frequency switching in accordance with the invention, not only canthese heavy costs be reduced but improved facilities provide fortransmission and interconnection. The Teed connection of subscribers toa commondistribution circuit or telephone main becomes possible andmethods of interconnection at the exchange lead to the reduction ofswitch contacts to a minimum. Indeed the total elimination of suchcontacts from the speech path is possible. At the same time the trunkingwithin the exchange is greatly simplified. The invention provides forthe extension of this principle to junction Working and to trunkexchanges in a manner which presents facilities not convenientlyattainable by conventional methods.

It will be appreciated that the invention can be applied for otherpurposes than telephony Whenever flexible interconnection of a number ofstations or locations is required as in telegraph or other signallingsystems.

These and other features of the invention will be better understood byreferring to the accompanying drawings in which:

Figure 1 illustrates in diagrammatic form a subscribers instrumentcircuit as connected to a telephone main consisting of two coaxialcables.

Figure 2 illustrates in diagrammatic form a carrier-frequency finderwhich hunts for a carrier frequency generated from a calling station.

Figure 3 illustrates an exchange-frequency selector by means of which aparticular carrier frequency having access to the wanted exchange or toan intermediate exchange may be selected or a wanted line selectorindividual to the calling exchange may be selected according as towhether an interexchange call is being set up or a local call.

Figure 4illustrates a wanted line selector.

Figures 5 and 5A illustrate a telephone main linking a plurality ofexchanges with repeatering arrangements.

Figure 6 illustrates in diagrammatic form the equipment according to theinvention as applied for setting up a connection between subscribers onthe same telephone main, it being lknown to set up such a connection byother means as in British Patent No. 455,279 above referred to.

Figure'? illustrates an alternative to Figure 6 using frequency ndingequipment only in the conversational route.

Figure 8 illustrates an arrangement by which access may be had to anumber of telephone mains from a multiple wanted line selector, and theWanted line selector of Figures 2 and 3.

Figure 9 illustrates suitable carrier frequencies for giving maximumefficiency in use of coaxial cables as telephone mains.

Figure 10 illustrates suitable frequencies for the various values of thedigits in a decade frequency generator.

Throughout this description it is assumed the transmission is effectedby amplitude-modulated double-side-band methods. This enables thegreater simplification of apparatus. The use of vestigial carriersingle-side-band Working would principally eect a saving in line plantbut would result in considerable complications of both subscribers andexchange equipment. Similarly frequency modulation although possiblewithin the scope of the invention appears to offer no advantages.

Throughout this description the selection of one desired frequency orchannel from the number that may be simultaneously present at one pointis effected by superheterodyne principles, i. e. by changing thefrequency of the desired channel to a uniform intermediate frequencywhich may be selected by an intermediate frequency filter whose designmay be uniform throughout the system.

This has been done for convenience in manufacture, but the invention isnot limited thereto and selection by means of a plurality of bandpassfilters alone or combined with frequency changing means may be employedif desired without departing from the invention.

Figures 1, 2, 3, 4, 5 and 5A represent a single connection and should beread together as shown in Figure 11.

A subscribers telephone instrument as illustrated in Figure l isarranged to transmit and receive signals including speech at carrierfrequencies. A number of such instruments of which only one is shown maythen be connected to a telephone exchange over the common guiding pathor telephone main represented by the coaxial cables CCIA and CCIB. Eachindividual subscriber is allocated a particular carrier frequency fortransmission on one cable sucn as CCIB and a further distinctive carrierfrequency for incoming signals over cable CCIA. Individual subscribersstations are T connected by spur cables to the telephone mainrepresented by CCI and CC2 from the exchange in a manner analogous tothe conventional distribution of electric power. It will be understoodthat the arrangement of two coaxial cables, one for Go channels and onefor Return channels is only given by way of example, as "Go and Returnchannels could be provided on a single cable using group modulationmethods to segregate the directions of transmission. It will be Aassumedthat 500 subscribers could be connected to each telephone main though itwill be appreciated that this figure could be varied upor down as may befound convenient or as may be practicable. It may be necessary to employrepeaters if the telephone mains exceed a certain length in which casewide band repeaters will be provided each adapted to amplify Go" andReturn channels appropriately. By arranging that all signallingfacilities from the subscribers stations such as calling, clearing,impulsing are at carrier frequencies, amplification is possible and thesignalling and transmission limits are therefore identical, andsignalling and supervisory facilities may be provided on a more generousand reliable basis than is possible within the limitations set by anindividual subscribers loop circuit in any conventional system.

Each subscribers instrument incorporates a number of thermionic valvesfor effecting modumenigeen tlation and;,dempdulation fof 1 the: :carriersignals and the :arrangement illustrated'isipurely .diagrammaticasamoredetailed `circuit is illustrated in azzcopendingapplication. -In thelight of recent-technical.developments the initial and'maintenance costof such' equipment :Will not be heavy, and fsizefand weight of thecomplete `equipment it is considered, would not present-fianysmarkedincrease .over :presentdesigns of apparatus. 'By usin`g=valvesiinsubscribers instruments,coupled with the provision of effectively:separate paths for.` each .direction of' transmission,1the subscriberis aprovidedwith va'consistent standard of speech transmission which isatlpresent-generally unattainable. f

In order to :obtain `power .for thexvalves 'for eachssubscribers'.linstrument, Y use may be 1 made of the telephonemain itselftofsuplply current from the exchange or any other convenientpower path.`VOtherscurces of power "can`be--provde'd if desired, lwhile by employinga special type 'of subscriber-fs :microphonesov r`that direct -currentneed` no longer `ybe f drawn from if the exchange andyor by usingminiature vvalves-the power con- SilmptOD of the instrument als-a wholecan be keptto--aereasonalole'figurev y i Referring vto the vequipment ofthesubscribers instrument# GGO I represents a -f crystal controlledoscillator `which normally feeds fthe mixers or modulators MIA Yand M IB. LFAI is an intermediate-frequency amplier andDI -is a detector. Thearrangement is such that when -the frequency allocated'to the Returnchannel ofthe substation is received from cable vCCIA such frequencycooperating with lthe frequencyv of CCO in themixerMIA'results in afrequency which Will-pass through IFAI'. Modulation of the frequencyreceived results in modulation of thefintermediate frequency throughIFAI being detected-'byDand-fed 11o-'either the loudspeaker or .receiverLSor--thefbell B yaccording to' the position ofA the switchhook'SW. LFAIis a lowfrequency amplifier-wh-ichl is adapted to be fed with speechcurrentsifromthe moving coil microphone MCM/or tube-caused to generateimpulses under control of the dialv switch DS, which speech currents orimpulses serve to modulate the frequency generated byCCOI by the aid ofmixer MIB, the resultant modulated carrier wave r'being transmitted oncable CC IB.

vThe operation is briefly as follows: y

Whenthe subscriber lifts Ihisreceivera potential is applied to causethe'mixer'MIB'to vbecome effective and cause va carrier frequencycorresponding to the frequency of the oscillator to-bet'ransrnitted'over coaxial vcable CCIB. This vcarrier frequency is induecourse detected by a frequency finder at the exchange asdescribedhereinafter, and a transmission connection set up to theimpulse responding equipment. `When thisis done a dial tone is caused tomodulate a carrier frequency removed from thefrequency of the outgoingcarrier vby axed amount correspondring tothe pass frequency of IFAI.vWhen this` number eitherY Vby interrupting-the emitted carrier or 'byapplying voice 'ffreqtlency'-=I,oi"ieI impulses theivtelephone `mainC'CIA and CCIB to ascei?V tainjifany..'substation on the telephone mainVin .calling .condition as determined bythe prese-y .to modulate the-scarrien The .latterffmethod fis preferred and maybeeffectedibyzcausingfthellcw frequency amplifier LFAI to oscillateintermittentlytby back. coupling. the amplifier! inaccordancewithftheimpulses to be dialled or nini-any othersuitable` manner. The inventionis lnot limv itedto: any .particular .method of transmitting the numberrequired fand push button -'selectiong'f of combinationsofvoicefrequency currents SVmaylloe used. The dialled. impulsesm'odulatefby means of Vmo'dulenr iM I Bthe carrier frequencyA` on l thecable'CCI-Bandvn'operates the impulsefresponding equipment in-accordancewith the destination of the caller. 'When the-Wanted subscriber repliesthe calling subscriber can speakby means ofthe movingcoilmicnrophonevMCM and resultant currentsasentover CC IB and can be heard vonf'theloudspeaker receiver-LS bycurrents received over CC I`A.

vIn #case-'the substation illustrated is called, -a carrierffrequ'encyis applied to CCI A" corresponding-to frequency :allocated to theReturnf channel'ffo'r theparticular'substation which frequencyv ismixe'dby vMI A and the output passes through IFAI to DI lwhich -inresponse triggers a gas dischargev relay (notshown) to causeithebellB'toringe. Alternatively the carrier frequency on- CC-IA-may be'v modulatedin accordance with ther ringingcurrent and the output vofV DI VVusedftocontrol-the-bell.

When the-subscriber answers-'he lifts the switch hook and thereby opensther bellcircuit and-connectsup the loudspeaker-LS. He can then Speak asvvbefore. Outgoing Yspeech currents modulate the carrier over CCIBandthe received speech currents VVVbeing mixed by MIA, detected byfDIvand receivedfbyv LS. K l Y Itf-willbe* understood that the precisemethofd of leffectingthe various operations described may beconsiderably modified without ldepartingfrorn the spirit oftheinvention. The 4power'supply for the equipment may be provided locallyor may be'fedV over `the coaxial cables from the exchange preferably inthe Amanner described in a copending application, in'

which asupply of alternatingv current for'heatin'g the valve cathodesand also` for ringing the'bell, is' transmitted over one coaxial cable4and direct current'for-high*tensionsupply to the thermionicvalves-overtheoth'er coaxial cable.

l For a `ring.- ingysignal-all Athat is necessary is to derive 'from themodulatonvof the incoming carrier Vvia de` tector DI anaudio-frequencyvo'ltage'to break down Va gas'discharge tube and allow alternatingcurrent to reach thebell B I.

AOtherarrangements will be readily understood by those versed in theart.

which 'lis Ato Iinteirnodulate with continuously varyingtfrequencies'anyfrequencies present on ence' of "aparrier `frequency corresponding tothefrequency of a crystal controlled generator CCOI iof a 'substation being.present on CCI B andr theabsence ofa carrier frequency on CCIApor-vresponding to 'the frequency "allocated 'to Vthe same substation for'the Return channel.

The frequency nder comprises essentially a superheterodyne receiver'-fwhich serves Vto -demodulatefa calling vsubscribers l`signalsEan'dlsa-v mixerlfor :modulator i'for: communication intheVA reverse`direction. 7^The receiver lincorporates ,a variable frequency oscillatorVFO2, a V mixer M2A, an intermediate frequency amplifier T ,FAZ and adetector D2A; The frequency of the oscillator is determined by a controldevice CD which may comprise, for instance, a reactance valve or similardevice which inturn is controlled'by a sweep oscillator SO, such assaw-tooth oscillator which is adapted to generate a controlvoltagc whichcauses the variable frequency oscillator VFVOZl to generate frequenciesvarying over the `Whole of the range of frequencies allocated to callingsubstations on thertelephone main CCIA. The frequency passed by IFA2corresponds to the difference between the carrierfrequencies of theGoand Return channels for each substation, ksuch difference being thesame for every substation connected to,` telephone main CCIA and CCIB.The output from the variable frequency oscillator VFO'Z is fed to amixer M2B through the back contact A2 of relay A to test point TPZ andalso to a mixer MZA through the back contact Bl of Vrelay B. Y A mixer Mwhich is provided individual to the coaxial line CCIA and common to allfrequency finders which may be required for the purpose of setting upsimultaneousv connections, is fed from a xed oscillator `F02 with afrequency say 2,000 cycles per second so that al1 the frequenciespresent on coaxial line CCIA are modulated and pass with the sum anddifference frequencies tc terminal 'IPZ and thence to M2B. Theconsequence is that as thefre- Yquency of VFOZ approaches one of thelmodulated carrier frequencies presenten TP2, the resultant differenceproduct from M2-C and M2B passes through low pass filter LPF2 lto thedetector D213 and tends to operate relay B. On the other hand as thefrequency of oscillator VF'OZ approaches that necessary to effectselection of a particular channel frequency on CCIB by mixer MZA andlter IFA2, the detection of the carrier of that channel by detector D2Acauses operation of relay A.

If frequencies are present on both the coaxial lines *CCIA and ACCIBcorrespondingl to a particular subscriber and indicating that such asubscriber is busy, then relay B will first, on account ofthe fringingaction of the side bands receive sufficient current from D2B as toeffect its operation. B on operation disconnects M2A and therebyprevents the operation of relay A when `the, frequency of the variablefrequency oscillator subsequently approaches a value at which relay Awould operate. On the other hand if there is no frequency present onCCIA corresponding to the frequency present on CCIB of a callingsubscriber, then relay B will not beV In ad` -pending upon the pass bandlter character-A istic of the f interrnediate frequency amplier IliAZf'Yf Y l ContactA! of relay A, on the operation of relay A,Y disconnectsthe sweep oscillator SO while Contact A2 disconnects VFO from the busyytone"circuit andfconnects.- the oscillator to` th transmittingmodulatorl or mixer M2D' thus placing a carrier of the frequency.allocatedv to the calling substation for receiving purposes on lineCCIA, and therebyy prevent other frequency finders Y from yconnecting upwith the calling substation, and preparing a transmission' path inthereverse direction. Y It will thusbe appreciatedthat the callingsubstation is now in association with terminals T2A'and T2B and thatsignals such asimpulsing or speech passing from the calling side ofthesubstation reached T2B in demodulated form, while signals suchasjspeech, tones or ringing current connected to terminal T2A, are modulatedbyMZD so asto ,reach the receiving side of the calling substationasmodulationsof the carrierfrequency as determined by vthe setting ofVEO2.y

jIt will be, understoodthat although relays A and B have beenillustrated and described as if theyA werev `of the usualelectro-'magnetic type, actually they can be replaced by electronicdevices for instance ythose `which operate to suppress the normalfunctions of various valves v-in the circuits by placing suitablepotentials on auxiliary electrodes. It should be explained herethatwhere electro-magnetic devices such as relays` or switches aredescribed herein, this has been done for simplicity in explanation forthey may with advantage be replaced by electronic devices performing theequivalent functions such as are now Well known in the art., In factelec-` tronic devices will be Aprobably desirable Vforhandling the highfrequenccies involved.

Relay A which is responsive to the presence of carrier frequency on CCIBcorresponding to that to which FVOZ is sent, is adapted to remainvoperated as long as the switch hook at the substation is raised and toreleasewhen the calling party hangs up. It therefore serves as a holdrelay and can maintain equipment of Figures 2 and 3 in operatedcondition throughout a connection and allow them to restore to normalconditions when the calling subscriber hangs up.

Contact D02 is controlled by a relay DA (Figure 4) responsive to theresponse of the called subscriber which relay is locked operated whenonce energised, over contact D-Ai in de pendence upon contact DAS ofrelay A.

The switching and impulse responding equipment required of course variesin accordance with the nizeof the exchange or network concerne'd, vandin the present description an ex--l ample is given'of a five-digitnetwork having a number'of exchanges of vup to 1000 line capacity.V'Connections can be set up between subscribers on the same oranothertelephone main linked to the same exchange, or can be connected over atelephone main to another exchange where they can" again be set up tosubscrbers connected to telephone mains associated with that'exchange.It will be assumed that two digits" are required toY determine theexchange required and three to determine the number required in the'selectedexchange sequently rit is necessary to provide an exchangefrequency selector such'as illustrated in Figure 3,? and then the calleither passes over lines LL3A and LLSB local to the exchange which mayconveniently'be a solid cored coaxialV cable, to a wanted line selectoras illustrated in Figure 4, if a call is to a subscriber connected to atelephone main associated-,With a calling exchange, or toJunction-recare; cables JeosA and JCCSB' lead-.-

ingtoawanted lineselector. at another exchangen Only a. single pairy ofjunctioncables are illustrated but as thisis a telephone.

or# exchanges.

main, it can comprise any number of pairs of cables,.. individual pairsAof cables having access toal-single exchange, r to'two or moreexchangesac'cordingf to theY tra-flic requirements of such.

exchanges.

In the,;descriptionA that follows' itl will bei' asf sumedc. that thetelephone main comprising the junction coaxial cables JCC3A. and JCCBBwill`V be:r employed totransmit to another exchange or exchanges on. all.carrier frequencies allocatedY to theY telephone.Y main except thosefrequencies which are allotted for'v local working. For in'- stance,there may be ninety-nine groups each including ,ten carrier'frequencies: allocated. for' interexchange workingY and one group ofiten carrier currents forl local working. It 4may* be desirable tousebandpass filters, to exclude from 'such'iy junction cablesv allfrequencieswhich they are not required tov transmit. l Y

Assdescribedfwith relation-toFigureiZ; the oute put from detector DZAVVleads to terminal TZB and. thence to; relay'AA whichis controlled inanysuitable way depending upony theform the'- impulses take (e: g.interruptions'ofy carrier or modulations by tone frequency) as` torespond intermittentlyas'v impulses are dialled.

As explained, relay A is energised to arrest.l

the finder. onV the frequency offthe calling substation and remainsenergised as long as carrier frequency.' isA being fed" to linea-fromthe calling substation. Relay A at contact A3 completes.

TS and HS form part of a decade-.frequency generator DFGB-as describedin ay copending application.. The first two switches are set' topositions corresponding to the iirstdigtsdialled,

and switch HS isadapted' to. hunt for a1free outlet. The means. by'whichthis. is achieved is given in the following description: When impulsesare. received; relay` AA operates.. inter mittently andr completes. atVAAI a circuitA for relay CJCv and the' magr'ietw MTT. through'- wiper.`of.:r steering switch SS in; normali position. CC'. `operates and at'ACCI completesv the circuiti for magnetr MS/ of steeringn switch. SS;MTT: re-

spends to the impulses andA steps the wipers of the tensor: thousandsswitch TTS" in accordance with the.v first. digit dialle'd. When thefirst seriesf of impulses. cease the-wipers of TTS rest' on'a contactcorresponding" to' the rstfdigit and relay-CC releasesi'shortly afteropening the circuitv cf MS at CCI to cause thewipers'of the steeringswitch SS to ad'Vance'one step' so that'. the irnpulsing circuit is nowconnectedv to themagnet MT of the thousands swit'c'lfil TS. The nextseries of impulses therefore operateMT and CC. MT advances the wipersofthe switch TS tothe 10 common difference of the frequencies on TTS is.ten timesl that of those on TS and 10() times that of those on HS. Y y

By intermodulating the frequencies vselected by TS and HS and filteringout their sum prod-- uct a wave is obtained whose frequency may haveanyof 100 equallyspacedvalues depending on. the positions of TS and HS, andby intermodulating this wave with the frequency selectedf by TTS andfiltering out .the sum productlagainf. an output wave is-obtainedwhose-frequency-may have any of 1000 equally spaced.values,.de pendy ingupon the settings of the switches. TIES', TS and IjlSindecafde'fashion.v -1

Difference products may be employed instead.. ofsum products ifpreferred. f* i y y When CC operatedfor the 'second dig-it, it come..pleted at C'CZ immediately the wipersTSv had:v left the normalposition acircuit for slowrelease.T relay T. Relay T on energising operated jconf"tact TI so'that when the wipers -of the switch-SS were steppedtothethird position` a circuit was` completed from the mixer M3D over TLfrontv contact, contactandwiper ofvswitch SS iny third. position to theoutput ofl thedecade frequency generator thereby completingl a testcircuit@ Con-A sequently the busytest circuit onlybecomes-effec-` tivewhen the relay CC de-energises-at.-the.endnfr the second digit.

The switch HS has a frequency connected to1 the normal contact whichwhen mixed witl-rthey frequency from switchv TS-.in M3B-givesfresultantfrequencies of which the sum frequencyfpasses through BPF3A to M3A-whereit mixes withrther frequency from switch TTSandther difference;(say) passes` through-BPF3B toi-the` testcircuitz; this latter frequencyis4 designed tocorrespond. to-the rst of the-selected group of'carrierfre;- quencies which lead to the wanted destination:A To the secondcontactof. the bank oil-ISewi-ped. over by the left-hand wiperv there isconnected?l up a frequency such that the resultant: frequency: appliedto the-testr circuit will. be equal` to. the-A carrier frequency ofVthesecondofthe--selected` group of` carrier frequencies andlso-on,.withthe. frequencies connected ,tov the third. and' Subsc--iquent contactsv of the same banlcr the. frequenE-g ciesA appliedto thetest circuit will correspond.`

ity of the switchHSVbutof course there may position correspondingtothethousands digit and n cies); y are made for: switches.v TSaIld' The? be.a, different number if desired, .provided that: the. whole ranger ofcarrier frequencies is.` no: greater than. the difference between thefre-- quencies selected Vby. thev switch TS ori-.successive-` steps. i

Terminals., T313v are multipled and are" con-f nected to the cableJ.CC3A and lead LL3A--re. spectively, overv which the. callisadva-nced-` .to` its destination. Terminals T30 are multipledL andconnected to the output of. a common-mixer- M3F which is fed vfrom anoscillator FFO with.. a fixed frequency say 500 cycles and'is also con,-nected to the terminals TBB soA that if there'jis any carrier frequencypresent on the cables or linesv with which these terminals areassociated,y 'then the frequency on terminals T30Y will be the sum' anddifference of that carrier frequency and:V the xed frequency.Iftherefore the first off thel selected groupv of carrier frequencies isbusy there will be an output from MSD correspondsA ing to the xedfrequency which' pass-ing tofdef tector DSA through thelow pass-lterLPFSwill. cause-the relayn GG tofoperate;l Relay GG vhat,v

`through its own Yinterrupting contacts and a maintaining circuit forrelayj'T at GG2. The

switch HS will therefore step and advance'thev wipers Vuntil a carrierfrequency is ,placed von the test circuit for which there is nocorresponding frequency on the cable or line being tested. 1 Y

If GG does not operate or after it has ceased to operate the circuit forrelay T is opened and consequently this relay will release after a shortinterval Yand at contact TI front contact transfers the rfrequencyoutput of BPF3Bv fromv MSD to M3C and( M3E rendering TSB busy withrespect to the particular carrier frequency found and MEueifective tomodulate audio frequency currentswith the desired carrier frequency,while M3C is rendered effective to demodulate modulated carrierfrequencies received over-the se(V lected channel.

If the digits recorded by TTSfand TS correspond toa local'call then theconnection will have been extended over line IL3B and LL3A to a wantedline selector as illustrated in Figure 4,

whereas if the digits correspond to a subscriber' in a distant exchangethen the connection will have beenextended over JCCSAandJCCSB to a`similar selector' in one or other of the exchanges as illustrated inFigure 5. Obviously the local linesfLL3B and LL3A will be designed 'tohandle the high carrier frequencies andV may inA fact be coaxial cableslocal to the exchange.

A'wanted line selector local to the exchange is'illustrated inFigure 4which comprises two elements, the receiving element which is a simpliedform of the equipment required for frequency finding as describedirrFigure 2, and a digitaccepting element which corresponds to Figure 3except that it functions to establish connections directto thesubscribers instrument.

Each'wanted line selector in a group is designed 'to accept signals Vatone particular carrier frequency only, which is hereinafter described asits acceptance frequency; and to return a correspondingsignal Vat afrequency which differs from its acceptance frequency by the constantintermediate frquency. The receiving element comprisesfmixers M4Bassociated with the line LLBB and'MaA associated with the une LL3A..'rresek mixersare so arranged Ythat normally theexed oscillator F04whose frequency differs for every selector in the group, is connected toM4A sothat, when an exchange selector such as illustrated in.

Figure 3 places a frequency onto LL3A corresponding to the acceptancefrequency of the selector, there will bea resultant intermediatefrequency which will be Iallowed to pass and be Y amplified byanrintermediate frequency amplimission ofthe hundred, tens and unitsdigits.

dialed froma calling substation. TheseY impulses are received by M3E(Figure 3) and serve; to rmodulate the carrier frequency connected toM3E and transmitted over LL3A to the wanted line selector in Figure 4.Here M4A provides an outputwhich will pass through IF4 and D4A" tocontrol relay BA. `Relay BA responds to these impulses at contact BAIcompletes the cir-A cuit for relay BC and for the magnet MHD of thehundreds switch HDS. The wipers of switch 12 HDS are accordingly steppedto a position corre-v sponding to the digit dialled. On vthe termina.-tion of the dialling of the hundred digit, relay BC releases and as ithad on operation prepared a circuit for the magnet MS4 of steeringswitch SS4, the magnet now releases and steps the wip'- ers of SS4 tothe second position. In this position the impulsing circuit is extendedto magnet MTN of the tens switch TNS so that the impulses correspondingto the tens digitA now serve to reoperate relay BC and to step thewipers TNS to a position corresponding to the tens digit. At the end o-fthe tens digit relayr BC again releases and the wipers of switch SS4 arestepped as before, this time to the third Sposi tion. In this position acircuit is prepared Yfor the magnet MU of the unit switch US andtheimpulses for the units digit serve to step the wipers of the switch USto a position correspondving to the units digit. At the end of theseries of impulses relay BC releases, the magnet MS4` steps the wipersof switch SS to the fourth position. Contact BC2 completed a circuit forrelay TA through the off normal contacts of switch US, when the wipersstepped off normal, while' at contact BCS a circuit is completed fr0mBPFIIB over off normal contacts of switch US to front contact TAI ofrelay TA to M4F. The switches HDS, TNS and'US form part of the decadefrequency generator DFG4 andare adapted to connect up frequenciescharacteristic of the digits recorded, which frequenciesare arranged tobe mixed by M4D, BPF4A, M4C'V and BPF4B as described in connectionwiththe decade frequency generator DFG3 of Figure 3, so that theircomponent values may be readily deduced. Preferably they are theidentical frequencies and conveniently come from the same source in theexchange, e. g. a master crystal oscillator. e I

M4D serves to mix the frequencies selected by the left-hand wipers of USand TNS so that the output includes the sum and difference ofv suchfrequencies. BPFIIA serves to select either the sum or differencefrequency, but not both, and pass it on to M4C. M4C is also fed by theleft-hand wipers of the hundreds switch HDS with a frequencycorresponding to the hundreds digit and therefore the output of M40consists of the sum and difference of the frequencies in-v coming toM4C, of which one is selected by BPF4B and fed .as described to M4F. Thenext operation will depend upon whether the channel selected representsa connection to an individual' subscriber or represents the firstchannel of a number of channels in a common group, e. g. leading to a P.B. X line. In the former case, if the line is busy there will be afrequency on the coaxial cable CC4A corresponding to the frequencyoutput of the decade frequency generator DFG4. The frequency on CC4A ismodulated in M4I-I with 500 cycles so that there will be an output fromM4F of 5000 cycles when the output of DFG4 is connected up by the backcontact of TAI. This will pass through the low pass filter LPF4A todetector D4B and will effect the operation of the relay G. Relay G atcontact G2 holds relay T operated and at contact GI connects busy toneto the return path at M4B where it modulates the carrier frequencygenerated by F04 and passes back for demodulation by M3C, IFA3, D3B ofFigure 3. It is modulated by M21) of Figure 2, to the carrier frequencyallotted to the calling substation for incoming signals to the callingsubstation where it is again mixed at MIA with the frequency of CCOI andthe output acetate wlchipasses: vthrough 117.0 detrlll. D1" S; demodulated; and; passesd through;vr the: operated.

switch` hookV contacts.V to. receiverl LST. toj advise caller that the;required,` subscriberl is engaged.

At; the `same timev relay-1v G- maintainsgtheeoperation of relay- TA1 toprevent; this relay; releasing. Theic'alling; subscriber; now hangs up`his receiver'l thereby releasing; relay A which f, results at contactA4;- in. homing circuits,- being completed successivelyv forfthe.switches SS;` HS, T Sf and-v TTS of' Figure 3. The,vv circuit o fl relay'1?v is opened; and all the" apparatus in .FgureS' is; re-- stored tonormal and is now in readinessforan other call. The release ofrelay Aalso venables the variableoscillator V-FOZ` to come againunder control:of sweep oscillator SO and` resume hunt Thecircuit of relay TA isopened and-this: ref- RelayAjB at -AB I f completessuccessivelayfafngzl, relay G release'and all apparatus in ligure 4- is restoredAtonormal in readinessfor another call.

thefother hand, if a called substation is free; then therewillbeno-carrier frequency cor` responding tothe Wanted subscribers lineon CCAA. and when .relay BC releases it opensv the circuit of relayI TAWhichaiter=an short interval falls back and connects output fromBPlil-B..v

over its. back contact TAI tomixers. NME. and

M4G.. Theeffect of acarrierv frequency of a Wanted. stationbeingconnected to.(Dif/"Iller11ev sults not. only in marking,h thisvfrequency'echannel. as, eng-aged; but. alsoin. circuits being con,-trolledlat-the .substation as described:V with relatiorrto YFigure: 1vforcalling, the Wanted substa-A` tion. ThisI isefected. bywconnecting to-mixer lM4Gr throughv contact DB3- aringing.l frequency (which. maybeinterrupted. if desired in a. char.-v acteristicringing. cycle)Which,. on demodulation. inthe. Wanted substation, will: cause thefbelltoy Ringing. tone. is connected at. this timev ring.. from. the. fourth.contact and. middle. Wiper of switch SS4 vover. back contacts TAZaI-1d..D.I..off`

relays Tl and D respectively.. WhenA the subscriberY answers; the.carrier.. frequency characteristic. of. the yWanted station. from thelocal oscil-l latonsimilar to.- CCOI (Figure-1), issconnected` to CCHBwhichv is` mixed V,by M4E` and the, resI sultant .passedthrough and..amplified by thefin-v termediate. frequency amplifier IEAB. and de`.i

tectedby D4D. .The presence. of. thisintermer.

diate frequency in D4D results in? theoperation of'relay D which bymeansof DAandDB causes a momentary voicefrequency signal.combmationrSpeech from the called` substation modulatestheV frequency o fj theAcrystal oscillator thereat whichV is miXed by MAE with'theoutput'supplied" from IBPIF4B`A and'` the output' which' pia-.SeesVthrough IFAB' isfdemodulated by D4D; .This output isvv in 'turnmodulated' by means. offMljthe carrier frequency determined by F06passes. over lines LL3B andv mixesl at M3C with the frequencyyA outputfrom BPFSB. rIhe output frequency pass'- I ing..,tliroughand.amplied`.'by IFA3"is detected..

65ncause relay to operate andthe operationfor;

non-operation of relay G willzresultin the opera-- 7 :untilltheilast hn'e ofthe group is reached? by.l D313 ,and audo--frequencyagainproduced... Which at M22 modulates -thefrequency assigned? to; thecalling substatien for' incomingfspeech as supplied- DyfVFOZ At the.VAcalling station'audio l frequency P currentsare again vproducedby MIA,

intermediate amplifier IFAI-, detector DI and areI fed'to thel receiverLS. Similarly, speech fromy thecalling subscriber is modulatedy by M I Bwith.; the carrier' frequency determined by CCOI, vFig-- ure` l-, mixedat MEA `with the the.r carrierA fre. quency; VFOZ andI the outputpassingthroughit IFAZ is' converted 'to audioA frequency by detector D2A Theseaudio frequency currentsinturn mix at `MSE with `the frequency fromvBPF3B and.

pass over LLSAWheretheyare mixed 'at-NMA with. thefrequency vFOII andthe resultant which passes through IFIIA is again converted toaudio-.freel quency by Dil-A. Thisaudio-frequency. output in turnmodulates. the frequency from BPFAB. at M dGthat is the carrierfrequency assigned tothe- Wanted station. At the Wanted station theincomingY carrier is` mixed with the frequency ofthe local oscillatorin` aY similar methodv tofthatde.- scribedf in connectionwith'thereceiving side yof the-substationV of Figurefl.

- When the connection Yis terminated the calling.. subscriber hangsupand releases the connectionr in Ithesame wayas. previously described.

If on theother hand the lcalled. line in the'rstv of a. number ofsubscribers in a common group such` as.P. B. X subscribers, thentherewill-be. a; mark-er frequencyy determined by- MOI connected to.testterminal- Tf1-4A A marker oscillator such.-

as. MOI M02 .and M03. is .provided for each chang-v nel of carrierfrequency allocated.. to a par-z. ticular P. B. X except the. lastchannely of the. group. The frequency of eachrnarkeroscillatorisconveniently removedby two kc. from-the. Return? frequencyof the channel it is to mark.

Whenthe decade frequency generator represented' by HDS, T'NS and USr andassociated.` equipment is settothe frequency of the channel. so.Vconnected, the mixer, MdFfiwill admit a beat oftwokc. to pass throughtheband-` pass filter BPFllC to thedetector DISC andwill cause: relaylH. to operate.v If the channel is busythe. relay G-v/ill also operate aspreviously described: l The combined operationof GandvI-I at contacts G2andi-l2y complete a stepping circuitof thef` magnet of the. uni-t.switch US which there. upon steps tor the next.v contact and causes.frequencyY to be put out. through BPFdB. corre.- sponding tothefrequency ofthe seconda-channel.. of the. group. Relays Gand-AH.v are.therefore adaptedto test as` before. The second andsubs.

.sequent channels ef a group, exceptthe last,v are.

marked byy M02, MO 3; with frequencies. which are two. kc. removedfrom.- the frequencies producedv from BPEQB bythe second andsubsequenirposi tions of the switch-US for the positionjustlde-iscribed.: Relay Hseirvesto see. if the-last channelvof.groupis. reached or1not, and relay G tol determinewhether vthe channeltested is. busy.v or. not. If itis thelastchannel of the. group. there.YW-ill' heno frequency of a marker oscillator to;r

tions described inconnectionwith ther-testingv for free and idlecondition of a single channel: Relay.: Heatlcontacizrhl,prevents busytonefbein'g given `I-n place?l off a large number of marker oscillylators ttienecessary 'marking' frequency may beJ derived fromja; commonfrequency multiplier or7 a'4 pulse generator, thesfrequency' desiredbeingf' fed to the busy test point TT4A.

'" Figures 5 and 5A illustrate how connection is made to variousexchanges from junction cables JCCSB and JCC3A` (Figure 3) and alsoillustrates the provision of suitable amplifying arrangements orrepeaters for each -way of working. The individual equipment for each ofthe exchanges in Figure employing frequency selectors would be identicalwith that shown in Figure 4 as regards wanted line selectors, and asFigures 2 and 3 for junction hunting and local selection of wanted lineselectors. The exchange illustrated in Figure 5A is assumed to be anexchange operating with mechanical switching and a description oftheequipment required is given hereinafter.

Referring'to the drawing, Ecc2, E933 and E324 represent exchangesconnected to the telephone 'main' represented by cables JCCBB and JCC3A;

MN2, MN3, MN4 represent matching networks between the exchange equipmentand the junction cables and RPI and RP2 represent repeaters, one foreach way of Working. The repeaters comprise wide band amplifiers capableof dealing with 'the full range of frequencies which is liable to betransmitted over cables JCC3A and JCC3B. Repeaters would be provided atintervals along the cable as may be found desirable. The cables joiningthe exchange equipment With JCC3A and JCC3B may deal either withincoming calls or outgoing calls or both as found convenient. Spurcables SCZA, SCZB; SC3A, SC3B; SCIIA, SCAB, which mayv also beV of thecoaxial type, are provided for connecting the exchanges Ez, Ex3, Exil tothe cables JC'CSB, JC'C3A via matching networks MN2, MN3, lVlIN4.

FigureA shows an extension of the junction cable to an exchangeemploying mechanical switches EXS. This shows both incoming "Go andReturn spur cables ISCSA and ISCSB and an outgoing Go and Return spurcable OSCSA and OSC5B separately for convenience although incoming andoutgoing spurs could conveniently be combined. The incoming spur cablesextend to a matching network MN5A and from these are extended toterminating equipment TESAV similar to that shown at the reception orleft-hand side of Figure 4, for each incoming channel of communication,that is to say the equipment equivalent to MdB, F04, MBA, IF4A, DllA, ABand BA the frequency of the oscillator F04 being in each caseappropriate to the respective channel. The impulsing relay BSAequivalent to BA is illustrated. Incoming and outgoing audio frequencyspeech pairs from TE5A are connected to a hybrid coil arrangement HCSAwhich has connection to a balancing network BNSA and a connection to themechanical switching equipment MSE5A by which acc'ess is had to thewanted subscriber represented by SSA. The hybrid coil interconnects afour-wire Y circuit and a bi-directio-nal two-wire circuit in a fmannerconventionally used in the telephone art. `Relay BEA controls theimpulsing into the mechanical switching equipment so that the operationof this equipment takes place in well known manner according to theparticular system in use.

For outgoing calls the mechanical switching equipment MSESB is takeninto use by a calling subscriber and connects either directly orindirectly to a hybrid coil arrangement I-IC5B. The outgoing Go andReturn circuits from the hybrid coil arrangementI-IC5B extend toequipment OGSB similar to that shown in Figure 3, a

balancing network BNSBbeing providledin'thev usual manner so that when aconnection is set up to HCSB the impulses dialled by the callingsubscriber are caused by the impulse repeater IRSB to control thesetting of the decade frequency generator in OGSB corresponding to DFG3(Figure 3) and thereby to select andl idle frequency channel allocatedto the exchange required and to extend this frequency through thematching network MNSB to the outgoing spur cables OGC5A and OGCSB. Theimpulse repeater IRSB serves to feed battery to the calling' subscriber.

It will be appreciated that in this Way it is possible to eliminate atleast one stage of exchange selection from the mechanical switchingequipment in that it is replaced by a frequency selector and this isVery convenient if eventually it is intended to convert a mechanicalswitching` exchange to a frequency switching exchange.

Alternatively one could have an individual outlet for each outgoingchannel of communication with the appropriate carrier frequency createdfrom a fixed frequency source somewhat similar to M3C, M3D and M3E,LPF3, DSA and GG of Fig-A ure 3, utilising relay GG to mark the testcontacts of the mechanical exchange selector, but as this equipmentwould Kbecome largely superfluous if the complete conversion waseventually to be effected, this alternative is not considered desir- 4able.

It will 4be appreciated that the invention is not limited in any way tothe particular layout described, but the principles of the invention canbe applied both to smaller and larger networks.' For instance, Figure 6illustrates the employmentj of the circuits described where only onetelephoneV main comprising coaxial cables CCSA and CCiiBl is required toconnect all subscribers, for instance, f

the equivalent of a 500 line exchange or less. In this case the exchangefrequency selector can be eliminated and the impulse responding side ofthe equipment of Figure 4 connected ldirect to the equipment of Figure2. Consequently the equipment is conveniently represented by frequencyfinder switch as FFSA, FFSB and wanted line selectors WLSBA, WLSGB.

Figure 7 shows an alternative for the Wanted line selector according towhich two frequency,

finders (FFTA, FF'la; FF'IB, FF'Ib) such as illustrated in Figure 2, areconnected back to back and the operation of the second frequency, finderis set by hunting for a frequency determinedby a decade frequencygenerator DFGT which in turn is set by the dialled impulses. The decadefrequency generator in this case is not required to supply a 'carrierfrequency throughout the conversation as the carrier frequency for thispurpose will be supplied by the variable oscillator v of the secondfrequency finder (FF1a,FF1b) when may be employed at any numericalstagein av connection.

If it is desired to cater for a network of up to 5000 substations, forinstance, which can conveni- I7 ently. be dealt with yfrom; a`singleexchange, it will' be necessary :to provide sax'five differenttelephone mains, veachconsisting of a.' coaxial cable pair to deal withthe traffic. AIn suchy acaso. in order to avoid using two. stages ofSelectors as in Figures-3 and e, it is possibleA to employ a multiplewanted line. selector capable;

.Thev arrangement'` employedlrnight Vbe similar I to that illustratedrinFigure, in which separate finders are provided foreach, .coaxial Cable Pfi? capable ofy dealing with 10.0.0'. suhstationsiiol instance, FFBAserves coaxial cable pair CCSAi, CCSBI, EFBB serves coaxial-pair.QSBAZ,;CC3B2, FFSS,v serves CCSAB: and; and'. so on. Eachfrequency finder; FFSA, FFBB, FFB@ etc., is connected to a multiplewanted lineselector (MWLSBA, MWLSEB, lVlWLSC). gin-ing access` t all thecoaxial cable pairs (CGSB LCGGVA |-;,v G0852y CCBAZ.; CCBBB, CGBAa andksonni/and thereby to allthe. vsubstations the: network. ,Each MWLS is*provided with iii/1e,y independent .output connections, one to. each:telephone .irl-ain, only one of which is rendered; effec-tive; 'at atime. In practice' this would4 involvezproilidnga each wanted lineselector withfveo each. of' the modulators MAE, Meer,MAFiillustrated-inlgure fi, 'ILheoutput side of theseufive, sets .ofimodulators. MilG would be pe.rmanently.- connectedfto the. live out-`going telephone mains andtheinput: sides. of theV five sets ofmodulators MIIE and M4F would be permanently connected to thenveincoming` telephone 'mainsan'd .their .test points; respectively, but atVthe .conclusion of the.impulses;onlyl one of.. the 've sets .will be..energised depending upon the impulses;dia1led;.;f.or instance,inaccordance .with theV iirst digit..y If .the number .of sub.-

It. will be appreciated y.that .the wanted line.

selector employedin Figure .6 would receive signals at audio-frequency,whereasvwhere therel are twostagesl of' switching it is essential thatthe second stage .should incorporate in its` input cir.- ouit aiixedfstageof frequencygthus accepting-sienalsv not' at audio frequencybut at an inputcarrier frequency characteristic of.' the particularwanted line selectorchosen, thus separating the variousconversationsienter'ing thesecond switching stage.'

It may be noted. thatthe large number ofoutlets of the wantedrlineselectorv enables la verylarge group to be used withoutythe'limitations oi mechanical type switches as. previously used. Thusit is quite practicable for a multiple Wanted line selector to respondto two digitsandto provide 50 outlets from each of these- 100 levels. Bya simplearrangement thesel 501 outlets may he tested for Vthe 'busycondition iii/fe :ata time thus greatly reducingv theinterval beforevthe first free.

wanted line selector is located kandseized.

18 a wanted 'line selector if this should be found desirable in any partof the network. It is also possible to use multiple exchange selectorsif desired and by the use of multiple exchange selectors as well asmultiple wanted line selectors, two ranks of selectors may be arrangedto deal with a very large network of a size normally employing four ormore ranks of selectors.

It will loeappreciated that as all the connections between an exchangeselecto-r and a Wanted line selector are arrangedto carry a. number ofcarrier channels of diierent frequencies it is possible as illustratedin Figure 3 to have any desired number of local connections-i and anydesired number of connectionsover the junction cables so that with verylarge'isolated exchanges the junction cables may be entirely eliminated.However, it is possible as illustrated in Figures 5 and 5A, for thejunction cables to have access to any number of exchanges and of coursethere may bein large networks a plurality of junction cables crtelephone mains required to deal with thetraiiic. |l-he method `ofdealing with such cables will he readily appreciatedgfrorn thedescription which has already-been given by those versed in the art.

It. will be noted that not only has all apparatus individual tothe-'subscriber at the exchange been eliminated, but also itis'notnecessary to provide junction relayv circuits or other lterminatingequipment provided that the telephone mains are repeateredv atA suitableintervals if they are suinciently long so as to avoid any transmissionor.l signalling limitati-ons over the-junctions. It will be' also clearthat the general principles may readily-be appliedto satellite workingand tandem working bychoosing'equipment hav-ing theappropriateffunctions; Y

It will be noted that a single telephone main from an exchange such asthat containing the equipment of- Figures 2 and 3- may be Teed into anumber of other exchangessuch as liht2 -,llrr3',` E504- of Figure- 5 andE5 ,of Fig-,ure 45A., suitable ranges of frequenciesout oi".thewhole-ra'ngewith which the telephoneV main dea lS being apportionedbetween theseexhanges in any Way desired.. l f

AS an extension of this it benoten that a number of Subsidiaryexchanges" connected to a telephone main may in .effectiveallowed-toshare common junction groups; to larger VVex-l changes connected tothesarne-rnain inthisycase a call from one such subsidiary exchange willseize the rst free'channelo the-common group, busy condition being;exhibited simultaneously at allexchanges having;V access toA thegroup.This facility might prove of- -ilnportant value orserving lightly loadedexchanges or Y,inschemesfor thev decentralised` controloftrunktrainen/.hereby commonV laccess 4tov economically large groups: of trunkcircuits would bez-offered toanumberofgde.-

centralised call collecting; vcontrol points. facility is notavailablefin conventionaljunction working, unless athird'.Wirefis-provded.

As deiined above the expression' telephone mains broadly- Goversallforms-oa guidinggpath for the transmission of carrier Vfrequencycurrents linking a Ypluralityl off Vstations more; than,

two. Thefinest.satectorygform for the invention isa telephone`11min..consisting oitwo. coaxial cables.

main is individual to anrexchange1the,;substations may begconnected lto.1the mairnby'4 shortfspur Each. main vmay be. subdivi'dcdl .orgbranchedctoocover aggeographicalarea. It4 thel mainY serves the purposeof interconnecting three ormore exchanges, eachexchange may be connectedto the main by similar short spur cables or a cable may actuallyV enter'each exchange. Possibly solidY dielectric cables to which T connectionsmight be made'by. means of a suitable tool would be preferable for'thespur cables while the telephone main itself Would'preferably be an airspaced coaxial cable to which the spur cables wouldfbe -connectedthrough suitable impedance matching networks, thatis to say, generallysimilar'tothe arrangement illustrated in Figure 5 except that in certaincases the exchanges are replaced bysubsta'tions.

i Distribution schemes for large exchanges may include cables includingeight or more pairs of tubes withinfone sheath; Such a cable servingsome ve000-subscribers would materially reduce ductcongestion in theneighbourhood of large exchanges. By the insertion of wide bandrepeaters atappropriate intervals (say three to four miles) telephonemains may be extended to any desired length without impairing eithertransmission or signalling. I-t will thus be appreciated that notechnical limitation therefore exists to the length of an exchange line,which is a serious limitation'in theY present automatic telephonesystems. Such repeaters require a power handling capacityrelatedA to themaximum trafc on a telephone main atA any particular time, but not tothe'ultimate number of subscribers connected thereto. The advantage of atelephone main when laid between three or more telephone exchanges isthat it may be possible to eliminate, in certain cases, the requirementof providing tandemworking. rl'ihe' use of telephone mains fordistribution to substations and for junction routes provides aflexibility impossible with individual paired cables and greatlysimplifies schemes for local line development. Y

GenerallyV speaking it is desirable to give to the exchange selectorsand vwanted line selectors theV availabilitycof 1000 channels since thisfits in best with the decimalsystem of numbering. Assuming a channelspacing of 8 kc. the total frequency range covered will be some 8megacycles. While there is no objection to the use of such' a frequencyrange withinrthe exchange itself itis probably un'econ'omic to attemptto transmit 1000 channelsover one telephone main since the frequencyrange and the total amount of `power'tovbe handled would render thedesign of the repeaters difficult. In such a case it is suggested thattwo or even four telephone mains carrying say 500 to 250 channels eachshould be composited on entering the exchange to produce an effectivesingle 1000 channel main. Such compositing would be readily effected byemploying groupmodulating and demodulating equipment to shift the entirefrequency range of individual mains to different allocations in theequivalent 1000 channel main as described below. The frequency nders maybe arranged to scan the entire range of 1000 channels or the range ofone telephone main only.

At the left-hand of Figure a simple arrangement for compositing thecarrier frequencies of two pairs of coaxial cables is illustrated.

It .has been .assumed that 1000 outlets are accessible from the exchangeselector of Figure 3. In practice this would be too many to be dealtwith by a vsingle pair of coaxial cables. The employment of groupmodulators GMSA and GM5B which are fed from xed oscillators.

FOSA andv FO5B serve to ensure, firstly, that 20?. the carrierfrequencies lof the' requisite range which are transmittedHtow'ardslJCCD through the wide pass band 'filter BPFSB by mixing withthe frequency of FO5B are lowered in frequency for passage throughJCCED. Secondly, carrier frequencies received over JCCSC are mixed byGMBA with a group frequency of FO5A and the sum frequency passed throughBPFSA to the terminals of the exchangeselector so that by thuscompositing" 500 channels from JCC3A and JCC3B and 500 channels fromJCCBC and JCCED, the full range of 1000 channels is obtained. Y

It will be readily appreciated that such an arrangement can be extendedto apply to three, four or more pairs of '..cables as may be founddesirable.

In view of the receivers used in the frequency finder equipment being ofthe superheterodyne type, it is desirable to prevent the acceptance ofimage signals and this may be attained by arranging that theintermediate frequency should not be less than half the total band widthof the system. The working frequency may then be so arranged that allimage frequencies lie outside the working frequency bands to which thereceivers are responsive. In order to facilitate repeater design it isalso desirable that the ratio of maximum to minimumfrequency toV enterone telephone main should not exceed two or three. l

A typical frequency allocation is shown in Figure 9a the subscriberstransmitting (or "Go) frequencies for 500 channels spaced eight kc.apart are allocated between four and eight megacycles, the correspondingexchange calling (or Return) frequencies being allocated between sixVand ten megacycles. The intermediate frequency in this case is twomegacycles. A smaller group of 500 channels may then be group modulatedas previously described to the ranges 12 to 16 and l0V to 14 megacycles'and then combined to give a joint frequency allocation for 1000channels as illustrated in Figure 9b; It will be noted that thefrequency of the frequency finder oscillator and the decade frequencygenerator is now required to be variable from six to fourteenmegacycles.

The use of an intermediate frequency as high as two megacycles maybe'avoided by the split allocation shown in Figure 9c, where thesubscribers Go frequencies lie between three to ve and seven Vto ninemegacycles while the calling or Return frequencies lie in the range fourto eight and the yintermediate frequency is therefore now one megacycle.Such telephone mains may be composited as before, to give thearrangement illustrated in Figure 9d for 1000 channels. In this case thedecade frequency generator has to cover the discontinuous range of fourto eight and ten to fourteen megacycles. This presents no dimculties,but the design vof a frequency finder to deal with the same broken rangeof frequencies may make it desirable, if this type of allocation isVemployed, to arrange for each frequency finder to scan five hundredchannels only, exploring the range Vof four to eight megacycles. y Y

Y The choice of frequenciesV for the' decade frequency generatorrequires to take into consideration the provision of simple Afilterswhich will readily eliminate all unwanted modulations produced. Thefrequency allocation table illustrated in Figure '10 shows how a's anexample, a decade frequency. generator :may 'be made to provideReferring to Figure- 4, vvitli' this allocation of frfq1.iencyy filterBPFIA is arranged to pass frequencies from about 16;-750 to'- about19,7000 kc. only, it will therefore transmit only the sumproduct ofmixer Mll'A- Whos'eoutput may vary between V17,160 and 17?;950 kc.FilterBPFA-B is' arranged toJ pass all frequencies below about 15,500kc. and-- thus Willtransmit` only the difiere'nc'e;product-osmiiier-MllaThispro'duct which is the output of the generator, is variableffrm'6,048 megacycles to 14,040 megacycles--in 1000 stepsv` of eightlkcz,corresponding tothe 1000` numbers from 11--1 to 000'.-

It Wi'llbe apprcci'ated that the `eliriiination of higherrorder'modulation products'- facilitates the use; of4 gas dischargef` sv'vitchingY inthe 'vv-ayreferred t'o previously, l 1

1. A carrier-frein-lerici; group-selector compris-- ing an incoming linehaving-go and return paths; niodulating-Y nlieans'i associatedr with'rthe go path ofr` said incoming line; demodulating means associatedWith;y the returnfpath of-y said incoming line; a transmissioninediumfora plurality of carrier-frequency chan-nels; such channels havingVseparategfo and return paths,

y said transmission medium beingv connected to said modulating anddemodulating means, testing means including a demodulator for connectionto the transmissionl medium,V impulse responding means connected tothego path' of said incoming line, frequency generating means controlled bysaid impulse responding means to cause the generation of a first carriercurrent having airequencyfcorresponding to the numericalsigninc'ance or"the impulses to whiclisaid impulse responding means responds, means forfeedingysaidrst carrier currenty to the demodulator lof the testingVvmeans to ascertain ii the channel of' `communication ofsaid transmissionmedium corresponding-to saidfirst ,carrierl current isbusy; meanscontrolled-1b57 said testing means' and said'frequencygeiierating means.if saidchanneljisfoundtobelbusy' '6o-cause.' the generation of a 'secondcar-rieti' current hayinga diferentfrequeneyfrom that first selectedwhich howfev'er is' also distinctive of the 'numerical'sig- 'nificanceoflthe impulses-received vby saidimpulse responding incansandtio-'substitute said second carrier current for said-fiirst carriercurreiituin saidfeeding means causing saidY testing'y means to make atest-ofa-second channel of communication of the transmission medium andmeans controlled by said testing means when a free Vchannel ofcommunication is found of connecting that selected source of carriercurrent which corresponds to the carrier frequency of 'the free channelto the modulating and demodulating means associated respectively withthe go and return paths of said incoming line to establish a callingstation for x controlling the setting upoff a `connection,- a calledstation` 'connected Lto another cgi-said' exchanges; a carrier..4frequency gfroupselectorincluding frequency testing means andademod'ulator associated'.therewithWl'iich inJ response to control' fromthe calling, device in.v

accordancelwiththe designation: of?- said other. exchange'huntsltonnelan'idl'e channel o'f com'- mu-nicationon said transmissionmedium,modulating means operated under further control: off signals vsent'v bysaid` calling line ineaccordance with the remaining part of thedesignation of the. called line to cause saidsig-nals tocmoidulatethecarrier wave of` thef selected channel andi switching devices l at: saidexchange; responsive; tothe modulated signals: to. extend a connection.,tothe called-qstaton. i f

'telecommunication system as claimed-, in

claim Sinfvvhich .the exchanges linkeclfbyssaidv transmission medium.consist, of. a: centrali; exe change and'.satellit'epexchanges r 5. 'A.telecommimication system: as..cilaimedg` inv claim 3 in' whichoireoisaiot` exchangesis an combination a. calling station a3 calledstation.

comorisnga cablesMfforf-extendm aconnection fromy said calling to said'called statim` acarrier frequency groupselectorforgsettingup-aiconnec--. tion under control of-` said calling station to. saidtransmission mediumincludng a modulatorfand demodulator for ygoandreturn` paths, testing means including a demodulator and means.controlled by signals transmitted from said calling station todetermine'the freguenciesof carrier currents to befappliedsuccessivelyto said testing f demodulator'kuntil. an available cllannell of com/- munication onsaid transmissonmedium is found andthereafter apply carrier frequency. fcurrent correspondiiflg.. tgSad.sleted lafililI to said communication means between said incomingv vpathand the selected channel of said transmismodulator and demo'dul'atorfrom the go" and return paths, toenable. 'further "signalsvtllsmitadfrom seid. allinasaiioni@ moderate Seid `selected` carriercurrent, means accessible toi' said transmission. medium and.'responsveto such signal modulated carrier waves transmitted thereoverto extend the connection to a called station and means operable when thecalled station replies for transmitting carrier currents in bothdirections over said transmission medium which carrier currents aremodulated and demodulated by said modulator and demodulator for the goand return paths respectively.

8. A telecommunication system as claimed in claim 7 in which one of thepair of coaxial cables is used for one direction of transmission and theother one of the pair for transmission in the opposite direction.

9. A telecommunication system comprising iny combination a plurality ofcallin-g stations, a plurality of called stations, a plurality of wideband transmission mediums each comprising a pair of Y coaxial cables, acarrier frequency group selector comprising testing meansa-nd ademodulator for hunting for .an available channel of communication Aonsaid transmission mediums and means for determining; thefrequencieswith'which the signalling circuits for calling stations are to bemodulated andthe transmission mediums over which the connection iste beset up and group modulating means at thecalled end of said transmissionmediums by which the selected frequencies .are arranged to` be shiftedso as to occupy frequency ranges distinctive of both the Afre-- quencyand the medium selected in a bandof frequencieswider than the band foranyone transmission medium. l j

10. AV. telecommunication system comprising in combination atransmission path having associated therewith modul-ating means by whicha groupof communication channels may be established thereover, eachcommunication channel having a carrier Wave for the calling direction ofcommunication anda second carrier Wave separated from saidrst carrierWave by ,a fixed amount for the called direction oi` communication, asecond transmission path also having associated therewith modulatingmeans by which a group of communication channels may be establishedthereover', leach communication channel havinga carrier Wave for thevcalling direction of communication and a second vcarrier Wave'separatedfromsaid first carrier Wave by a fixed amount for the calleddirection of communication, carrier frequency group selectors includingfrequency testing equipment with -demodulators for interconnecting achannel' of the iirst group with .an available channel of either! 'groupcomprising modulated Vmeans by which .the calling carrier WavemiX'e'dWith -aWave of suitable frequency to give arsultantwave lyingwithin' a standard-'frequency vband-"and further modulatingI'nea'ns'b'y` lW-hic'h-said resultant wave is mixed with awave of suchfrequency as to give a resul-tantv Wave of fa frequency corresponding tothe carrier Wave frequency off-the' called direction of communie-ationof the Wanted communication channel. i f

ill., Atelecommunication system as claimed in claim in Which-the'waveWith-in the standard frequency ban-d is at audio-frequency.

l12. A carrier-frequency group selector associated With'aY transmissionpath and compri-sing selecting.means`,'an oscillator adapted to generateonef'oia `ijn'nnbelr of waves ofmdierent frequencies las determinedVbysaid yselecting means, testing means including a demodulator by whichsaid generated wave is caused to cooperate Withwaves on saidtransmission path'a'nd responding means which operatejif a Wave of afrequency correspending to that determined-by saidselecting e means' is'already vreselit:011-v 'the :iialslmsoil path to cause said oscillator.to generate succes-A sively waves having frequencies related tothefrequency lrstdetermined until avvaye is generated having a frequencylfor which there is no counter- Dalt Y011th@ transmission patha f I f13.A- frequency-selector associated with a transmission path to which anumber of Vcall stations are 'connected and comprising selecting means,an oscillator adapted to generate, as -determined Vby.sai-,dse1ec1ngmans,10ne ma number of Waves of `different frequencieseachl char,-

acterizing a called station, testing vmeans including .a dernodulatorand reslziondingvnieansk which operate according as to whethenacurrenthaving a frequency characterizing the station called isdetectedby saidtesting means on said transmission path or not,- anumbertoij communication REFERENCES CITED.

The following references are of record in the file of this patent:

UNITED STATES. PATENTS Number Name Date :1,568,194 Smythe Jan. 5, 19262,064,907 Green Dec. 22, 1936 2,084,903 Frink June 22, 1937 2,107,168Tid-d Feb. 1, -1938 2,114,718 Levy Apr. 19, 1938 '2,137,023 Monk Nov.1'5, 1938 2,143,5'63 Levy Jan. 10, 1939 2,202,474 Vroom May 28, 19402,270,385 Skillman Jan. 20, 1942 2,298,409 Peterson Oct. 113. 1942V"2,324,394 Holden July 13, 1943 2,337,878 Espenschied Dec. l28, 19432,345,048 VHubbardet al. Mar. 28, 1944 2,887,018 Hartley Oct. 16, 19452,408,085 Meacham Sept. 24, 1946 2,408,462 Wise Oct. 1, 1946 2,409,063Ostline Oct.V 8, 1946 2,440,239 Almquist Apr. 27, 1948

